1. Field of the Invention
The present invention relates to printers such as ink jet printers having up to multiple print heads, and more particularly to alignment of one head to others thereof such that printout for each print head superimposes accurately and with good quality.
2. Description of the Related Art
Printers such as ink jet printers have become an extremely popular format for achieving high quality computer printout at low cost. Such printers print an image on a recording medium by uni-directional or reciprocal back-and-forth movement of one or more print heads across the recording medium. In the case of ink jet printers, a printed image is formed by ejecting small ink droplets from a print head in predetermined patterns onto the recording medium. The print head is mounted on a moveable carriage which provides right and left reciprocal movement at high scanning speeds across the width of the recording medium, while the recording medium is slowly fed in the lengthwise direction.
Recently-introduced printers, particularly ink jet printers, have multiple print heads, such as two or more print heads mounted on the reciprocating carriage. The print heads may be identical to each other, such as dual black or dual color print heads which increase black and white or color printout speeds by up to a factor of two. Alternatively, the print heads may differ from each other, such as a black print head paired with a color print head which provides good color reproduction without sacrificing print speed for black and white documents. As a further example, some ink jet printers are equipped with one full color print head paired with a photographic-density color print head, so as to achieve high quality photographic-like printout.
One complication introduced by providing printers with multiple print heads is the need to align printout for one of the multiple print heads to all others of the multiple print heads. Without alignment, mechanical manufacturing tolerances would cause printout from one print head to be mismatched in either or both of the vertical or horizontal direction relative to printout from others of the print heads.
Moreover, printout from even a single print head often differs when printing in forward and reverse directions. Thus, alignment of a single print head to itself is sometimes needed, so as to align printout in the forward direction to printout in the reverse direction.
Some existing multiple head ink jet printers utilize a manual alignment technique in which predetermined patterns are printed and the computer user is asked to respond to questions concerning quality and appearance of the printout. Such techniques are not generally satisfactory, in that they cause needless user confusion, result in inconsistent alignment accuracy, and inevitably complicate use of the printer.
The assignee of the present application has recently described a technique for automatic alignment of multiple print heads in an ink jet printer, in which an alignment sensor is mounted on the carriage together with the multiple print heads. According to this technique, automatic alignment is achieved through printout of predetermined patterns, automatic sensing of printout results, and calculation of alignment parameters. See U.S. application Ser. No. 08/901,560, "Auto-Alignment System For A Printing Device", the contents of which are incorporated herein by reference as if set forth in full.
In one example of an automatic alignment procedure described in application Ser. No. 08/901,560, each print head is caused to print a highly repetitive pattern, with the phase of the pattern (i.e., the starting position thereof) being shifted gradually for one print head relative to the other. The superimposed printout of the two print heads exhibits a correspondingly varying density signature, which varies in correspondence to the gradual phase shift, and which is sensed by the alignment sensor. Perfect alignment between the print heads is that point at which the printed density pattern is lightest, as sensed by the alignment sensor. This technique is explained in more detail in connection with FIG. 1.
Shown in FIG. 1 is the alignment pattern printed by each of print heads A and B, together with the result of superimposition of the alignment patterns, so as to align print heads A and B in the horizontal direction. As shown in FIG. 1, alignment pattern 11 for print head A consists of repetitive printouts of vertical columns of pixels 12 arranged three columns wide, followed by three columns of no pixels (i.e., white space on a paper recording medium). Likewise, alignment pattern 14 for print head B consists of repetitive patterns of three vertical columns of pixels 15 followed by three blank columns. However, for print head B, at each of areas I through VI, the starting position of the pattern is shifted horizontally by one pixel. Thus, as shown at area II, the starting location of pattern 15 is gradually shifted rightwardly by one horizontal pixel 16. The width of each region is approximately 60 patterns wide.
The result of superimposition of the alignment patterns is shown at 17. In region I, the patterns from print head A and print head B overlap completely, resulting in a printed output 19 that appears as dark vertical lines three pixels wide followed by bright white lines also three pixels wide. At each of regions II through VI, the alignment patterns for print head A and print head B overlap to increasingly lesser extents. In particular, at region IV, the alignment pattern does not overlap at all, resulting in a printed output which appears to be solid black space. Because approximately 60 patterns are printed in each region, an alignment sensor 21, whose alignment face is approximately 40 or 50 pixels wide, would sense the pattern in area I as having a lightest printed density relative to the pattern in area IV which would be sensed as having a darkest printed density. Perfect horizontal alignment between the print heads would then be calculated as in region I.
In like manner, alignment between the print heads in the vertical direction can be obtained through printout of vertically-arranged repetitive patterns with the phase of the pattern for one print head being shifted gradually relative to the other. Such a pattern is illustrated in FIG. 2.
The alignment technique above is extremely advantageous since it is entirely automatic and provides good alignment results without the need for user intervention. On the other hand, and particularly when alignment is performed using low-grade paper as the recording medium, practical difficulties limit the ability of such an alignment technique to provide alignment down to .+-.1 pixel.
In particular, as shown at the inset in FIG. 1A, when printing alignment patterns on low grade paper, ejected ink bleeds from the ideal borders of the alignment patterns into adjacent regions. For example, as seen at 22, ink from an ideal alignment pattern bleeds into regions which should remain white, thereby decreasing the ability to distinguish between a lightest superimposed pattern and a darkest superimposed pattern.
Furthermore, as shown at 19 in FIG. 1, because alignment patterns for head A and head B are completely superimposed, region 19 receives 200% ink quantities. Such a large amount of ink in so small an area causes cockling or other warping of the paper recording medium resulting in an inaccurately printed alignment pattern.
FIG. 3 shows another difficulty in producing accurate printouts of alignment patterns, relating to variation in carriage speed during printout. Shown in FIG. 3 is a graphical representation of carriage speed versus horizontal position across the recording medium. As shown in FIG. 3, the carriage speed ramps up from a stand still position toward a target scanning speed, but exhibits overshoot and other ringing properties which are most significant at the beginning of the scan but which continue to a smaller degree even after the target scanning speed has been reached at 31. Since print heads A and B are both mounted on the same carriage but with a horizontal offset therebetween, it is clearly necessary for the carriage to move horizontally in order for print head B to print superimposingly over the same position as printed by print head A. Thus, when print head A prints at position X, the carriage may be moving at slightly higher speed 32 than the target scanning speed 31. Later, when print head B prints at position X, the carriage may be moving at a slightly lower speed 33 than the target scanning speed 31. This difference in carriage speed when printing the alignment pattern for head A relative to the alignment pattern for head B leads to further inaccuracies in the superimposed alignment pattern result, and leads to further decreases in alignment accuracy.
Finally, alignment accuracy is also affected by the ability of sensor 21 to distinguish between a darkest printed density area and a lightest printed density area. However, as shown in FIG. 4, the difference .DELTA. between a darkest density area and a lightest density area is often quite small. FIG. 4 is a graph showing variation in printed pattern density as sensor 21 scans across regions I to VI. The density range shown in FIG. 4 varies from around 0 to 255, and the readings in FIG. 4 are obtained by density conversion of an analog-to-digital converted output from sensor 21 as it scans across each of regions I through VI. As can be seen in FIG. 4, alignment sensor output for region I is different than that for region IV (which represents perfect alignment) by only an amount .DELTA. which may be around 15 to 20 counts out of a possible 256. Much less of a difference is evident between regions III through V. Altogether, the small value of A, and the small change from region to region, make it difficult to detect which region represents the best alignment. This difficulty is compounded when the effects of noise are superimposed on the graph shown in FIG. 4.